Diamond cutting tools are hard and can be made with very sharp cutting edges suitable for ultraprecision diamond machining such as diamond turning, diamond milling and diamond grinding. Diamond turning, also known as single-point diamond turning (SPDT), is an ultra high precision machining technology that uses single-crystal diamond tools with precision control systems to produce optical quality surfaces (e.g., for surface finishing) and/or precisely remove materials (e.g., for micro- and nano-machining) with sub-nanometer level surface finishes and sub-micrometer form accuracies.
One major drawback of diamond machining is that it can only machine a small group of materials that are called diamond machinable materials including some nonferrous metals (e.g., copper, aluminium, electroless nickel (a Ni—P alloy with a phosphorus content more than 10%)), some polymers, and some crystals such as silicon. Diamond machining cannot machine many important engineering materials, which restricts it to be a universal ultraprecision machining process and therefore restricts its applications and market expansion. For example, the metals that are not diamond machinable include ferrous alloys, stainless steel, titanium and its alloys, and nickel and its alloys. Non-diamond machinable materials can extremely quickly wear out diamond tools so that the diamond machining of those materials is not considered cost effective. For instance, turning steel will wear a diamond tool 10,000 times faster than turning brass. Even for turning diamond machinable materials such as copper and electroless nickel, a diamond tool will eventually be worn out although its wear rate is much lower than that for turning non-diamond machinable materials.
Diamond tool wear is a complex phenomenon that is not fully understood yet. Several tool wear mechanisms have been proposed, including 1) adhesion and formation of a built-up edge; 2) abrasion, microchipping, fracture and fatigue; 3) tribo-thermal wear; 4) tribo-chemical wear (e.g., oxidation, diffusion/dissolution, catalyzed graphitization, and carbide formation); 5) thermal-chemical wear, and 6) tribo-electric tool wear. It is not uncommon that multiple mechanisms contribute to diamond tool wear simultaneously under some conditions although there is usually a dominant mechanism for a certain tool/workpiece combination and a certain cutting regime.
Chemical reactive wear (chemical reaction between diamond and workpiece) is one significant tool wear route in diamond machining. In this wear mechanism the surface layer of the diamond tool is transformed from the diamond form of carbon to the graphite form of carbon with the transformation catalyzed by a workpiece such as steel and/or removed due to diamond carbon oxidation by the reduction of the surface oxide of a workpiece such as copper.
Paul et al.'s research on various pure metal elements provides a structural explanation of the chemical wear of the diamond tool resulting from graphitization (see Ed Paul, Chris J. Evans, Anthony Mangamelli, Michael L. McGlauflin and Robert S. Polyani, “Chemical Aspects of Tool Wear in Single Point Diamond Turning,” Precision Engineering, Vol. 18, No. 1, pp. 4–19 (1996)). Their hypothesis ascribes this chemical wear to the presence of unpaired d-shell electrons in metals. In other words, if a metal has no unpaired d-shell electrons, it is diamond machinable. In contrast, if a metal has unpaired d-shell electrons, it can catalyze the transformation of diamond carbon into graphite carbon.
This hypothesis explains very well why nickel is not diamond machinable while electroless nickel is diamond machinable. Nickel has two unpaired d-shell electrons. Therefore, it is not diamond machinable. Electroless nickel actually is a nickel-phosphorus (Ni—P) alloy. The p-shell electrons in phosphorus atoms form chemical bonds with the unpaired d-shell electrons in nickel. Therefore, electroless nickel becomes diamond machinable as it has fewer unpaired d-shell electrons than pure nickel.
Kohlscheen et al. also found that although titanium (Ti) is not diamond machinable TiNx is diamond machinable by adding nitrogen (N) into Ti (see J. Kohlscheen, H. R. Stock and P. Mayr, “Tailoring of Diamond Machinable Coating Materials,” Precision Engineering, Vol. 26, No. 2, pp. 175–182 (2002)). This is because covalent bonds form between Ti and N so that unpaired d-shell electrons in Ti are tied up by N.
The above two examples reveals a strategy for tailoring a non-diamond machinable material into a diamond machinable material by adding a suitable element into the non-diamond machinable material for bonding unpaired d-shell electrons in the material.
Shimada et al proposed another chemical wear mechanism in which diamond carbon on the tool surface are removed due to carbon oxidation by the reduction of the surface oxide of the workpiece (see S. Shimada, T. Inamura, M. Higuchi, H. Tanaka and N. Ikawa, “Suppression of Tool Wear in Diamond Turning of Copper Under Reduced Oxygen Atmosphere,” Ann CIRP, Vol. 49, No. 1, pp. 21–24 (2000) and S. Shimada, H. Tanaka, M. Higuchi, T. Yamaguchi, S. Honda and K. Obata, “Thermo-chemical Wear Mechanism of Diamond Tool in Machining of Ferrous Metals,” Ann CIRP, Vol. 53, No. 1, pp. 57–60 (2004)). For example, when a copper workpiece exposes to an environment containing oxygen such as in a normal diamond turning environment, a layer of copper oxide will form on the copper surface. When the surface diamond carbon of a diamond tool contacts the copper oxide, the carbon is oxidized by the copper oxide while the copper oxide is reduced by the carbon. Although copper is a diamond machinable metal, it still wears the diamond tool at a low chemical wear rate via this wear mechanism. For machining diamond machinable materials such as copper, it is desirable to have a lower chemical wear rate so that a diamond tool can ultraprecisely machine larger copper components or more copper products before it wears out. Diamond machining copper in a reduced oxygen environment will extend diamond tool life.
Several methods have been tried for reducing the chemical wear rate of the diamond tool for machining materials that chemically react with diamond. U.S. Pat. No. 4,563,114, issued on Jan. 7, 1986 to John M Casstevens, teaches a method for reducing diamond tool chemical wear during the machining of steel in a gaseous hydrocarbon-saturated atmosphere such as in a methane environment. The gaseous hydrocarbon inhibits or prevents the conversion of diamond carbon to graphite carbon at the contact point between the diamond tool and the workpiece so that the diamond tool wear is lessened. The reported machinable steel area with this method reached 3 square inches increasing from 0.3 square inches in a normal air environment. Therefore, this method does not significantly increase tool life and can be only used to machine very small steel parts. In addition, this method needs to confine the diamond tool and the workpiece in an enclosure filled with methane. This method is not only impractical for industrial manufacturing, but also not health and safety friendly.
As the rate of a chemical reaction is temperature dependent, Ralph L. Lundin, Delbert D. Stewart and Christopher J. Evans disclosed a method for the reduction of diamond tool chemical wear by chilling the tool and the workpiece to very low temperatures with liquid nitrogen in U.S. Pat. No. 5,103,701 issued on Apr. 14, 1992. The chemical reaction between the diamond tool and the workpiece is retarded at low temperatures so that diamond tool wear is reduced on non-diamond turnable materials. However, up to the present time, the only test results indicated that the tool wear was rather low for machining less than 1000 square millimeters (1.55 square inches) of stainless steel (see C. J. Evans, “Cryogenic Diamond Turning of Stainless Steel,” Annals of the CIRP, Vol. 40, No. 1, pp. 571–575 (1991)). The cryogenic method is also not practical for industrial manufacturing because of the difficult integration of a complex chilling system into a precision diamond lathe. In addition, it is difficult to achieve high precision components with this method due to the presence of severe temperature gradients.
Applying an ultrasonic vibration to the diamond tool for machining non-diamond machinable metals such as steel is another method proposed by T. Moriwaki and E. Shamoto (see T. Moriwaki and E. Shamoto, “Ultraprecision Diamond Turning of Stainless Steel by Applying Ultrasonic Vibration”, Annals of the CIRP, Vol. 40, No. 1, pp. 559–562 (1991)). In addition to the one-directional ultrasonic vibration assisted diamond turning, Toshimichi Moriwaki, Eiji Shamoto and Makoto Matsuo invented a two-directional or elliptical ultrasonic vibration diamond turning in U.S. Pat. No. 6,637,303 issued in Oct. 28, 2003. The idea of ultrasonic vibration assisted diamond machining (UVADM) is to apply a vibration to the diamond tool in the direction of cutting with ultrasonic frequency so that the diamond tool cut a workpiece intermittently in each vibration cycle. This feature brings several advantages over normal diamond cutting. First, UVADM effectively decreases the contact time between the diamond tool and the workpiece during machining. Secondly, UVDAM allows the cutting fluid to more effectively cool and lubricate the diamond tool and the workpiece. Lastly, UVADM greatly reduces the cutting force. Therefore, shorter contact time, lower average cutting force and better cooling effectiveness contribute to reduce tool wear and extend tool life.
UVADM is a mechanical solution to diamond chemical wear. As the diamond tool still contacts the workpiece, the chemical reaction is inevitable although the diamond tool wear rate is slower. It was reported that the maximum machinable area of hardened stainless steel was only 17 square inches with elliptical ultrasonic vibration diamond turning.
UVADM also has other disadvantages. An extra, high-cost ultrasonic vibration system needs to be installed on a normal diamond turning lathe. The quality of the ultrasonic vibration system is crucial, which determines the surface finish quality of the workpiece to be machined. It is not always easy to precisely align the vibration direction with the cutting direction. The resulting misalignment along with the other imperfect mechanical factors of the vibration system may cause the tool to vibrate laterally. The lateral vibration not only increases surface roughness, but also limits cutting speed, i.e., cutting efficiency. Because the diamond tool periodically cuts the workpiece, vibration marks form on the surface in addition to feed marks, which deteriorate surface finish quality and increase surface roughness. To reduce the influence of vibration marks, to increase surface finish quality, and to increase tool life, the cutting speed and feed rate have to be set as low as possible, which again decreases cutting efficiency. In addition, with this method the diamond tool has to be attached to the ultrasonic vibration system so that the tool cannot be spun and the workpiece has to be spun for the cutting purpose. It is sometime desirable to spin the diamond tool (e.g., flycutting) while holding the workpiece stationary. However, UVADM cannot be applied to this configuration.